Human Immunodeficiency Virus (HIV, also known as HTLV III and Lymphadenopathy virus or LAV) continues to be spread in epidemic proportions throughout the world. HIV is believed to be the causative agent of Acquired Immunodeficiency Syndrome (AIDS) and AIDS-Related Complex (ARC), a prodrome of AIDS. Although the AIDS epidemic may have leveled off in the United States and the Western World, it continues to increase in third world countries, especially Africa.
At the present time, the identification of individuals infected by HIV is based on the detection of antibodies and antigens specific for HIV in body fluids. Antibodies normally do not develop for at least a week after exposure to the virus and may not develop for up to 6 months or, in rare cases, two years. Thus, there is a long period of time during which it is not possible to determine if infection by HIV has or has not occurred. Furthermore, there are many circumstances when the detection of HIV-specific antibodies cannot be used to evaluate whether an individual is infected by HIV.
Present screening tests for identifying HIV-infected individuals are based on the detection of HIV-specific antibodies using various ligand-based techniques, such as an enzyme linked immunosorbent assay (ELISA) as originally developed by Gallo and co-workers in 1984 (Gallo, R. C. et al., Science 224: 500-502, 1984). Because the diagnosis of HIV infection is so devastating, after an initial positive screening test, HIV infection is confirmed by Western Blot analysis for HIV-specific antibodies. A new method for this confirmation is the demonstration of the presence of the viral genome by Polymerase Chain Reaction (PCR) analysis (DeRossi, A. et al., Lancet 2: 278, 1988). This confirmatory, follow-up test is 10 times more expensive than either original test. In addition, PCR analysis requires more expertise in its performance, more sophisticated materials and is subject to artifacts if not carefully controlled. Therefore, PCR will probably seldom, if ever, be used as an initial screening test for HIV infection.
Furthermore, there are circumstances when the detection of HIV-specific antibodies is not informative and there is no prior scientific basis for a screening test presently available. Non-limiting examples of circumstances where this situation prevails or may prevail are set forth below.
For example, babies of HIV-infected mothers may have maternal-derived HIV-specific antibodies but only 30% to 50% are truly HIV-infected and progress to develop AIDS (Blanche, S. et al., N. Eng. J. Med. 320:1643-1648, 1989). As with adults, PCR and/or tests for HIV-specific antibodies are used in an attempt to discriminate children with maternal-derived antibodies from infected babies. A positive result by either technique is perceived to be definitive of infection, but a negative result is not definitive (Borkowsky, W. et al., Lancet 1: 1169-1171, 1987). Repeat testing is usually necessary until: (a) maternal antibodies disappear; (b) clinical symptoms develop; (c) T-cell marker abnormalities are documented; or (d) the newborn reaches 2 years of age. Furthermore, both of these techniques are relatively expensive and, for reasons unknown at the present time, PCR does not always identify the presence of the HIV viral genome in the first few weeks of life. In summary, at the present time there is a gap in our ability to determine which infants born to HIV-infected mothers are truly HIV-infected and which only have maternal-derived HIV-specific antibodies.
There are at least 6 situations when the determination of HIV-specific antibodies is not informative. In each of these circumstances, a new diagnostic method is needed.
1. As illustrated above, to screen newborn infants of HIV-infected mothers.
2. Immediately after the occurrence of high risk events, such as an accidental needle puncture from an HIV-contaminated sample, prior to the development of HIV-specific antibodies.
3. As an adjunct for the screening of whole blood for transfusion or manufacture of blood products. Although all donated blood is currently screened before it is transfused, the screening procedures used are for HIV-specific antibodies employing ELISA and Western Blot analyses. If the donor has not yet developed antibodies (i.e. seroconverted) and is HIV-infected, the disease can be transmitted (Ward, J. W. et at., New England Journal of Medicine 318: 473-478, 1988). In addition, recently recipients of organ donation have been found to be HIV infected. This occurred because the organ donor was HIV-infected and had not yet developed HIV-specific antibodies and was not known to be a member of one of the high risk groups (male homosexuals, intravenous drug users and hemophiliacs) for HIV infection.
4. To identify HIV-infected patients after immunization. If an HIV vaccine is approved and used, it will not be possible to distinguish those individuals who have been vaccinated from those who are truly HIV-infected. Alternate methods of screening the blood supply will also be needed.
5. To identify new therapeutic agents and to monitor their efficacy in therapy as a surrogate end point. Currently, all such monitoring must be done by analyzing a patient's immune functions (see below).
6. As part of research studies to identify HIV-infected individuals of other species. For example, vaccine testing is now starting in animal models. After vaccination, the animals are exposed to HIV and the infection is monitored over time. However, because all infected animals have antibodies induced by the vaccine and may develop other antibodies from HIV exposure, HIV-specific antibody-based techniques cannot be used to monitor the efficacy of the vaccine. Similarly, new therapeutic agents will be tested in animal models and, again, efficacy cannot be determined by simple ELISA, or enzyme immunoassay (EIA) or other tests for HIV-specific antibodies and antigens because all of the animals will be infected and have such antibodies and antigens.
Progression to AIDS in individuals with HIV infection is presently evaluated by monitoring lymphocyte T-cell markers. Lymphocytes with T-4 antigens (also known as CD4) are present in normal (uninfected) individuals in higher concentrations than corresponding cells with T-8 antigens (also known as CD8). At the time of infection, and for part of the asymptomatic period, T-cell markers are usually within normal limits. Only as HIV infection progresses to AIDS does this ratio reverse; when the number of T-4 cells fall below 400 per ml, most patients have clinical features of AIDS. A second gap exists in the knowledge of events and markers for progression to AIDS during the asymptomatic stage of HIV infection. Currently, high levels of .beta.-2 microglobulin have been associated with poor prognoses (Mossi, A. R. et al., AIDS 3: 55-61, 1989) whereas elevated levels of urinary and serum neopterin (a folic acid metabolite) have been found in individuals with HIV infection and the levels increase as AIDS develops (Fuchs, D. et al., Immunology Today 9: 150-155, 1988). Thus, although the complete causal relationship has not been fully elucidated, measurement of either component has been proposed to serve as a prognostic marker for AIDS, but neither can be used for the early detection of HIV infection because the above-mentioned changes in concentration occur only a significant time after infection.
What is needed in the art are new methods for determining whether a patient is infected by HIV in the situations mentioned above in which the analysis of HIV-specific antibodies is not informative.
Therefore, it is an object of the present invention to devise methods for identifying individuals who are HIV-infected.
It is a further object of the present invention to devise methods to identify HIV-infected individuals in situations where the detection of HIV-specific antibodies is not informative or practical.
A still further object of the present invention is to provide a diagnostic agent which can be employed as a surrogate end point for monitoring the therapy of HIV-infected individuals.